Evidence for crustal extension and inversion in eastern Tasmania, Australia, during the Neoproterozoic and Early Palaeozoic

Abstract

The island of Tasmania in southeast Australia consists of a number of stratotectonic elements. The relationships between these
elements are largely obscured by younger cover of the Tasmania Basin, which contains extensive dolerite sills that limit the
ability of potential field techniques to map basement. Therefore the development of a robust tectonic model for Tasmania has
been inhibited. To assist in the development of a tectonic model, a deep seismic reflection program undertaken offshore around
the entire island was designed to map the large-scale structures of Tasmania at depth. The airgun seismic energy was also
recorded at a number of seismographs deployed across the island, allowing low resolution 3D tomographic imaging. Short reflection
profiles were recorded onshore across structures which could not be imaged by the offshore profiling. This paper focuses on
eastern Tasmania. In the seismic sections, the Proterozoic basement in the southeast is mostly featureless, except for large
rotated blocks with weakly reflective boundary faults, indicating extension of the Tyennan Element by block faulting. The
deposition of the sedimentary succession of the Adamsfield - Jubilee Element was related to this extensional event. In the
northeast, a reflective lower crust is interpreted to represent thrust slices of previously highly extended continental crust
and possibly fragments of oceanic crust. The Early Paleozoic sedimentary succession of the Northeast Tasmania Element formed
across the inverted margin. The apparently complex geology of eastern Tasmania therefore fits into an extensional model where
continental extension eventually led to the formation of very thin continental crust and possibly oceanic crust to the east.
The extension was probably related to Late Neoproterozoic extension recorded elsewhere in Australia. The region was subsequently
shortened, probably in a northeast - southwest direction, with most shortening accommodated in the seismically reflective,
probably oceanic part of the crust, and little or no shortening in the block-faulted, and extended continental crust.